OPU-1
UNIVERSAL POWER-LINE CARRIER SYSTEM TYPE OPU-1
GENERAL DESCRIPTION - Rev 7 (May 2019)
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The following information describes the different functions performed by the pilot channel.
Automatic Gain Control (AGC)
The equipment supervises at all times the level of the pilot signal received in each one of
the channels. The amplitude of this signal, once digitized, is used to carry out the automatic
gain control of each channel. Thanks to the use of digital processing techniques, it is also
possible to compensate for a level variation of the receiver outputs, which is caused by the
presence of noise in the pilot channel.
Link synchronization
It is possible to work plesiochronously, that is to say, each terminal works with its own
master clock or in a synchronized way, that is to say, each terminal uses its internal
oscillator as the master transmission clock and synchronizes its reception with the other
terminal using the pilot received. This synchronization is completely digital.
Telephone signalling
Modulating the pilot signal by frequency shifting at a maximum rate of 50 Bd, corresponding
to 25 impulses a second, permits telephone signalling to be transmitted.
Internal data transmission
The supervision of the system is carried out by transmitting data through the internal
communication channel at a rate of 50 Bd. The transmission is interrupted when telephone
signalling appears and is resumed when it has no transitions.
Furthermore, data transmission is periodically interrupted in order to carry out the link
synchronization.
Noise spectral density
The system estimates the noise spectral density from the measurement of the noise power
in the band of the pilot tone. Assuming that this density is constant in the whole 4 kHz
channel, the signal-to-noise ratio is independently calculated for each of the channels.
The value thus determined is compared with pre-set thresholds in order to block the
previously-programmed audio-frequency outputs and to deliver an excess-noise alarm.